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Carbohydrates - Naming and classification

Video transcript

okay so the term carbohydrate refers to a chemical compound made up of carbon atoms that are fully hydrated so carbo for carbon and hydrate for hydration or or water and because these biological molecules are our hydrates of carbon you can find them fitting into the general formula a C so a number of carbon atoms so in for just kind of a generic number of carbon atoms and then a matching number of water molecules so h2o s usually in the end the exact same number as as your carbon atoms and because all of these carbons kind of have an Associated water you can think of this as being essentially a 1 to 2 to 1 ratio of carbon hydrogen and oxygen now when we have one of these carbohydrate molecules we call it a monosaccharide so mono saccharide and monosaccharide essentially means one saccharide and saccharide is just a synonym for carbohydrate so saccharide is actually it's it's derived from the Greek word for sugar so you might hear a single carbohydrate referred to as a simple sugar but in all these instances what we're talking about the same kind of molecule and these molecules that that were calling carbohydrates they do some pretty incredible and and pretty hugely necessary things in our bodies and in most living things for that matter and maybe one of the most familiar of these tasks is the fulfilment of our body's energy source so carbohydrates fulfill our body's energy needs and then the main energy source for metabolism in our bodies is glucose so glucose and I bet you've heard of glucose before if you might have heard it in the context of checking blood glucose levels for people with diabetes but glucose is a monosaccharide made of six carbons and you might also be familiar with kind of the structural rigidity of cell walls in plants and that rigidity comes from the rigid carbon backbone of several carbohydrates linked together to form the polysaccharide so polysaccharide several saccharide several carbohydrates linked together and we call it cellulose so cellulose is is the polysaccharide that the carbohydrate that makes up the structural backbone of cell walls and then of course one of the most beautiful carbohydrate rolls in my mind is the use of ribose which is a five carbon sugar that supports the transcribed products of our genes in RNA and so you might have caught on that in all three of the carbohydrates that I just mentioned we see the ending os-- OSE and we see that in glucose cellulose and in ribose and that's because os-- is the suffix for sugars and and there are actually two prefixes that help us further break down the naming of these compounds and so the first prefix that we're going to considers how many carbons are in a chain so the number of carbons that are in the chain for this molecule so for example I'm going to draw glyceraldehyde which is kind of generally considered to be the simplest carbohydrate and it looks like this and if this carbonyl group right up here was just an another hydroxyl group this would be glycerin and three carbons with three hydroxyl groups but instead it's an aldehyde and so this this molecule is for that reason named glyceraldehyde and the aldehyde is our functional group and so if we're going to count how many carbons are in this molecule we'll start with with our functional group carbon this carbonyl carbon up here we've got one two three carbons and glyceraldehyde and so there are three carbons and for that reason we would call this a try oh s--- try four three and again us as our as our suffix for sugar and if we added a fourth carbon we would call it a Tetris so four carbons in na and a carbohydrate chain is a Tetris and then we add a fifth carbon and that would be a pentose so pentose four five and if we added an additional carbon we would have six carbons and that would give us a hexose hex being the prefix for six and I mentioned before when I was talking about the energy source of our body that glucose is actually actually a six carbon carbohydrate and so as an example of a hexose I'll draw a glucose here so this this carbon chain there's six carbons we've got one two three four five six carbons and this is a hexose called glucose now in the case of glucose the functional group is an aldehyde just like our glyceraldehyde so the functional group up here is an aldehyde but what if we make it a ketone you see we can actually make it a ketone and still retain that 1 to 2 to 1 ratio and it brings us a kind of another pretty popular hexose called fructose so this is another hexose it still has that 1 to 2 to 1 carbon hydrogen oxygen ratio but instead of having the aldehyde functional group it has a ketone functional group right here and so that brings that kind of the second naming prefix we have to indicate whether we're working with an aldehyde or a ketone so glucose would be more accurately referred to as an aldohexose and that aldo is a reference to the fact that the functional group in this carbohydrate is an aldehyde and fructose then on the other hand let me write fructose down so fructose fructose is a ketose exos and again that the keto is a reference the fact that the functional group here is a ketone and then if we want to just kind of exhaust this kind of second prefix idea going back up to glyceraldehyde which we said was a triose because the functional group is an aldehyde this would be an aldo triose so aldo aldo triose so we name based on length of the carbon chain the number of carbons that are in the chain and in the and the functional group that's in our carbohydrate and the last kind of major component of naming is the stereochemistry at the highest numbered chiral center so again we start with the carbonyl carbon and if we use a Fischer projection like we did with glucose and then we go to the highest chiral center which would be this last one and we decide the stereochemistry of that of that chiral carbon and just kind of as a shortcut with Fischer projections if the if the kind of highest substituent and in this case the hydroxyl group is on the right hand side then it's an AR stereochemistry and if it's on the left side it would be an L sera chemistry so let me try to make that a little bit easier and I'll just kind of redraw glyceraldehyde a nice small molecule as a Fischer projection so we've got our aldehyde and then we've got kind of our next two carbons in the chain three carbons and we have one chiral Center in in this carbohydrate this one right in the middle here and our Oh H group you can see is on the right hand side so this is an R configuration and for carbohydrates it was actually a lot of the naming is associated with the the guy Fisher who invented these Fischer diagrams and but he decided since it was an R and and the Latin for right-handed it kind of is is Dexter we assign a D to this configuration and then if we kind of drew this in a mirror image and we drew the in the in tumor and we had our aldehyde carbon up here and our last two carbons and their hydroxyl groups now we see that the the chiral carbon the highest numbered chiral carbon has a its primary substituent on the left side at the bottom of this Fischer projection and so this would be assigned an L which is a little bit easier and so as an example I guess going back to the the glucose this would be AD aldohexose because that the hydroxyl group is on the right-hand side of this molecule and again with our fructose same thing that the last chiral chiral carbon has the Oh H group on the right hand side so this is a D keto hexose now before I move on I want to allow you to review Fischer diagrams because I know I just kind of blazed through it and and really they're important especially with carbohydrates because stereochemistry becomes quite important in the biological implication of carbohydrates so I included a great video by Jay with Khan Academy on Fischer projections and it's actually the video I used to learn about them so I'd encourage you to kind of pause here for a second and give that video a watch on Fischer projections and get real comfortable with with absolute configuration and then we'll move forward with it with the discussion of carbohydrates